This invention relates to an improved conductivity monitoring system or arrangement for monitoring the conductivity of a fluid flowing into and through a conductivity cell.
In blood dialysis, blood is passed through a dialysis unit on one side of a membrane of cellulose or the like, and dialysis solution is passed across the other side of the membrane, containing a sufficient concentration of salt to render the dialysis solution generally isotonic with respect to the blood. During dialysis, unwanted material such as urea, creatinine, and some water pass through the membrane from the blood into the dialysis solution, so that the dialyzer serves the function of the natural kidney in many important ways.
There are many different designs of apparatus for supplying the dialysis solution to the dialysis unit. One well known design is the RSP Dialyzer sold by Travenol Laboratories, Inc., of Deerfield, Illinois. Other dialysis delivery systems which are available provide a lower quantity of dialysis solution to a Kiil-type, flat plate dialyzer or the like, in which the dialysis solution typically passes through a dialyzer in a single path, then is discarded. In the copending applications of Willian J. Schnell and Ludwig Wolf, Jr., Ser. No. 519,731, filed Oct. 31, 1974, entitled "Swirling Flow Bubble Trap", and of Wendel V. Ebling, Rene G. Lamadrid and Earl G. Phillips, Ser. No. 519,730, filed Oct. 31, 1974, entitled "Device for Separating Low Density Materials Such as Gas Bubbles from a Liquid, and the Use Thereof in a Dialysis Delivery System" various improvements for dialysis systems also are disclosed.
The conductivity monitoring system of the present invention is particularly applicable for use with the improvements and in the dialysis systems disclosed in these two copending applications, as well as in other types of dialysis systems. Further still, the conductivity monitoring system is applicable for use in various other applications, for monitoring the conductivity of a fluid.
More particularly, the conductivity monitoring system of the invention includes at least three electrodes which are disposed with the housing of the conductivity cell, into and through which the fluid to be monitored is caused to flow. One of the electrodes is a common electrode and is positioned closer to one of the other two electrodes. The electrodes are energized by means of an oscillator which, in the illustrated embodiment, has an output frequency of 500 Hz. The output of the oscillator is coupled to the electrodes through a transformer. The common electrode is coupled to one terminal of the secondary of this transformer and the other two electrodes are coupled to its other terminal. Resistance means are connected in series with the respective ones of these two electrodes, and effectively function as constant current sources when the electrodes are energized. The conductivity is measured between the wide-spaced pair of electrodes, i.e, the common electrode and one of the other electrodes, and between the closer-spaced pair of electrodes, i.e., the common electrode and the other one of the two electrodes, and the difference is taken as the actual conductivity. With this arrangement, changes in the conductivity as a result of foreign matter collecting on the electrodes and the housing over a period of time are compensated for, since the difference in the conductivity between the two pairs of electrodes remain the same, even though the conductivity between each respective pair may change.
The primary winding of a second transformer is connected across the resistance means connected with the respective electrodes and provides a difference voltage at its secondary winding which represents the difference in the conductivity between the two pairs of electrodes. This difference voltage, after being amplified, is sampled with a phase sensitive sampled and hold circuit, and the sampled signal is coupled to indicator means to provide an indication of the value of the conductivity of the fluid and/or to activate a high level or a low level value of conductivity alarm.
Temperature sensing means also are provided for sensing the temperature of the fluid, and the output signal thereof is utilized to provide a temperature compensated conductivity signal. Detection means also are provided for detecting and indicating the failure or inoperability of the temperature sensing means.
The transformers, in addition, provide isolation between the conductivity signal and the electronics of the system.
Accordingly, it is an object of the present invention to provide an improved conductivity monitoring system or arrangement for monitoring the conductivity of a fluid flowing into and through a conductivity cell.
Other objects and features of the invention will be apparent from the description below.